Windows 8.1 build 9391 has reportedly shipped to OEMs ahead of a public preview at Build 2013 conference

From Neowin:

WinBeta (via @windows4live) is reporting that build 9391 has been released to Microsoft's OEM partners for testing as a "Developer Preview." Not much is known about build 9391, as even screenshots haven't been leaked yet confirming it's existence. Win8China.com reports that this version of Windows 8.1 will not likely be the one that Microsoft unveils at Build, backing up claims by prominent Microsoft watchers who suspect that other builds will be compiled between now and June.

When Windows 8.1 becomes available to the public, it is expected that it will be released via the Windows Store and will come with a whole host of changes to the core operating system, possibly even including the return of the start button.